The invention relates generally to an improved overcoating material for protecting various heat sensitive record materials.
Specifically, the invention relates to an improved overcoating composition which is capable of protecting an underlying heat sensitive record material from the deleterious effects of (a) ultraviolet radiation, (b) physical abrasion and (c) certain chemicals while at the same time preventing excessive abrasion and wear of the thermal printing head used to form the desired printed images on the underlying record material.
As will be explained herein, the invention is particularly applicable to commercial labeling processes wherein machine readable indicia (e.g. bar codes) are thermally printed upon a heat sensitive paper. Accordingly, the invention is described herein with reference to such applications. It must be appreciated, however, that the invention has broader applicability and may find utility in connection with virtually any thermal or pressure sensitive printing process. Examples of other thermal label/tag printing applications wherein the present invention may be useful include, but are not limited to, laboratory strip chart recorders, electrocardiography, data processing, facsimile transmissions, pressure sensitive business forms, etc.
Many types of thermal printing papers or "heat sensitive record materials" are known in the prior art. Typically, such record materials comprise (a) a base sheet of paper or other "base material" and (b) a filmlike heat sensitive "record layer" formed thereupon. The "record layer" generally contains dispersions of reactive chemicals which are capable of reacting with one another to form a desired colored or darkened image in response to the application of heat and or pressure. Thus, when a thermal printing head applies heat to the heat sensitive record layer, the desired printed image will be formed therein.
The particular types or classes of reactive chemicals contained in the heat sensitive record layer generally dictate the type and density of color image formed therewithin. With respect to commercial labeling applications, two general types of color forming chemical systems have been employed.
First, many heat sensitive record materials used in commercial labeling applications have record layers which contain at least two reactive chemicals--a "color former" and a "dye precursor". The color former and the dye precursor are capable of reacting with one another to form a desired colored image. The color former and the dye precursor materials may be separately microencapsulated or otherwise separately contained so as to prevent casual mixing with one another with resultant premature reaction. When heat is applied, however, the separately contained dye precursor cand color former will melt or otherwise flow together so as to immediately undergo the desired color forming reaction.
Many specific types of dye precursors and color formers have been employed in the heat sensitive record materials of the prior art. In most applications, it is preferable that the color former(s) and dye precursor(s) be inherently colorless, pale or white in color prior to undergoing their color forming reaction so that the record material will be appropriately light in color. Accordingly, a group of light colored alkaline dyes known as "leuco" dyes are frequently employed as dye precursors.
A comprehensive listing of prior art dye precursors is set forth in the disclosure of U.S. Pat. No. 4,484,204 entitled "Heat Sensitive Record Material" at column 3, line 21-column 4, line 4 and such listing is expressly incorporated herein by reference.
Likewise, many types of color formers have been used in conjunction with the various dye precursors. Color formers known in the art include various phenolic compounds, inorganic acids, organic acids and other materials capable of undergoing free radical reactions with one or more of the above-described dye precursors.
A list of exemplary color formers is set forth in the above cited U.S. Pat. No. 4,484,204, at column 4, lines 5-36, and is also expressly incorporated herein by reference.
In addition to the dye precursor/color former chemical systems employed in some heat sensitive record materials, others have employed metallic salt/color former chemical systems. The advent of these metalic salt/color former systems came about due to the fact that many of the previously used dye-based systems were not suitable for machine reading using near infrared scanning equipment. Thus, the metallic salt/color former systems have the advantage of being readable by commercially available infrared scanners.
Metallic salt coatings, while exhibiting enhanced readability by infrared means, tend to be more expensive than dye-based systems. Also, the metalic salt based coatings as well as the leuco dye based systems, are known to liberate Na+ and/or Cl- ions which have degratory effects on the thermal printing heads. Thus, it is desirable to provide a smooth overcoating which may be applied over top of such metallic salt containing record layers so as to prevent excessive abrasion, wearing or degradation of the thermal head.
Regardless of whether dye-based or metallic salt chemical systems are used to form the image within the heat sensitive record layer, all of the heat sensitive record materials known to date have exhibited certain drawbacks. One major drawback associated with such materials is that they are not suitable for long term archival applications. The non-archivability of these materials is generally owed to one or more of the following shortcomings:
1. Poor resistance to physical abrasion; PA1 2. Poor resistance to chemical and/or substances; PA1 3. Poor resistance to ultraviolet radiation.
Additionally, as noted above, certain heat sensitive record materials--especially those employing metalic salt based chemical systems and/or phosphorescent materials--are known to cause abrasion and damage to the thermal printing head.
The poor resistance to physical abrasion exhibited by the prior art heat sensitive record materials becomes a problem when direct pressure or friction is applied to the record material. Such direct pressure or friction will cause darkened areas or scuff marks to form on the record layer. Such darkened areas or scuff marks may obscure any printed matter contained thereon.
The poor resistance to chemicals exhibited by the prior art heat sensitive record materials often presents problems when certain plasticizers, organic solvents, detergents, oils, amines, esters and the like come in contact with the record material. Plasticizers, organic solvents and some detergents are known to cause darkening of the record layers while certain amines and esters are known to cause quenching, fading or lightening of any printed images contained thereon. The above mentioned effects of certain detergents on heat sensitive record materials is particularly problematic in commercial shelf labeling applications wherein thermally printed labels are applied to high use areas such as the exposed edges of grocery store shelves. In such applications, the labels (and the neighboring shelftop) may be frequently exposed to washing solutions which contain detergents and other chemical materials. Thus, it is desirable that any heat sensitive record materials employed in such applications be protected from the deleterious effects of such detergents or cleaning solutions.
The poor resistance to ultraviolet radiation is generally manifested as a "photodegradation" of printed images previously formed on the thermally sensitive record material. Specifically, the reactive chemicals of the record layer generally form their printed images by way of generally weak bond. When UV radiation is applied, such gives rise to an actinic reaction whereby the bond energy of the printed image is overcome so as to result in degradation of fading of the image. Photooxidative type degradation centers initially on the actinic reaction of the ultraviolet energy on the photoexcited chromophores which give rise to the colored image. A continuous chain reaction may thereby result, causing breaking of bonds, cross linking, chain branching and/or free radical production. As a result, the colored images previously formed on the record material become faded or completely degraded.
Various attempts have been made to overcome the above-described drawbacks associated with heat sensitive record materials. One possible means of overcoming such drawbacks is through the application of a protective coating over the thermally sensitive record layer. However, the conventional application of solvent carried polymer coatings to such heat sensitive record materials has been found to cause severe darkening when applied to the standard record materials. Likewise, thermally cured coatings are not useable because the heat required to bring about the desired curing of the coating also causes undesired darkening of the record layer. For these reasons it is generally accepted that coatings intended for application to heat sensitive record materials should be (a) curable by nonthermal means and (b) free of volatile organic solvents.
In view of these limitations, it has previously been proposed to utilize certain radiation curable coatings as protective overcoats on various heat sensitive record materials. Such radiation curable coatings offer the advantage of being curable in the absence of heat and are generally devoid of volatile organic solvents or other chemicals which can cause darkening of the record materials.
"Radiation curable" coatings presently fall into two basic categories--those which are curable by ultraviolet radiation and those which are curable by electron beam radiation. U.S. Pat. No. 4,484,204 (Yamamoto, et al.) describes the application electron beam curable coatings to heat sensitive record materials for the purpose of improving the chemical and/or abrasion resistance thereof. While the application of these electron beam curable coatings may indeed improve the chemical and/or abrasion resistance of the underlying record materials, such coatings will do little to resist the effects of ultraviolet radiation as no reflective or UV absorbing materials other than pigments are disclosed. While some pigments may, due to their opacity, inhibit the transmission of ultraviolet radiation they have the attendant effect of adding color to the overcoat and may, when present in high concentrations, obscure any underlying printed images. Thus, because these prior art electron beam curable coatings fail to provide adequate ultraviolet resistance, they do not render the record material suitable for archival applications. While U.S. Pat. No. 4,484,204 does also describe the application of one ultraviolet curable coating to a heat sensitive record material (comparison Example 6, Col. 14, lines 14-22), the ultraviolet curable coating described therein is purportedly inferior in many respects to the various electron beam cured coatings to which it was compared (Table I).
Thus, there remains a need for improved radiation curable coatings which are capable of providing strength, chemical resistance, abrasion resistance dimensional stability and enhanced thermal head life while also protecting the underlying record material from the deleterious effects of ultraviolet radiation within a specific or known wavelength range.